Hexokinase I is one of four mammalian hexokinase isozymes. It is the pacemaker of glycolysis in brain and the red blood cell. Hexokinase type I differs form hexokinase type II in that the product inhibition can be reversed by physiological levels of inorganic phosphate. These properties of hexokinase type I may make it a critical regulator of glycolysis in brain tissue and erythrocytes. The crystallographic investigation of human brain hexokinase type I will deepen our understanding of the catalytic and regulatory mechanisms of hexokinases from mammals.